Bi-polar border region in piezoelectric device

ABSTRACT

An acoustic device includes a foundation structure and a transducer provided over the foundation structure. The foundation structure includes a piezoelectric layer between a top electrode and a bottom electrode. The piezoelectric layer has an active portion within an active region of the transducer, and a bi-polar border portion within a border region of the transducer. The piezoelectric material in the active portion has a first polarization. The bi-polar border portion has a first sub-portion and a second sub-portion, which resides either above or below the first sub-portion. The piezoelectric material in the first sub-portion has the first polarization, and the piezoelectric material in the second sub-portion has a second polarization, which is opposite the first polarization.

RELATED APPLICATIONS

This application claims the benefit of provisional patent applicationSer. No. 62/779,605, filed Dec. 14, 2018, the disclosure of which ishereby incorporated herein by reference in its entirety.

FIELD OF THE DISCLOSURE

The present disclosure relates to piezoelectric devices that employpiezoelectric films and in particular to such devices that have abi-polar border region.

BACKGROUND

Acoustic resonators, and particularly bulk acoustic wave (BAW)resonators, are used in many high-frequency communication applications.In particular, BAW resonators are often employed in filter networks thatoperate at frequencies above 1.5 GHz and require a flat passband, haveexceptionally steep filter skirts and squared shoulders at the upper andlower ends of the passband, and provide excellent rejection outside ofthe passband. BAW-based filters also have relatively low insertion loss,tend to decrease in size as the frequency of operation increases, andare relatively stable over wide temperature ranges. As such, BAW-basedfilters are the filter of choice for many 3rd Generation (3G) and 4thGeneration (4G) wireless devices and are destined to dominate filterapplications for 5th Generation (5G) wireless devices. Most of thesewireless devices support cellular, wireless fidelity (Wi-Fi), Bluetooth,and/or near field communications on the same wireless device and, assuch, pose extremely challenging filtering demands. While these demandskeep raising the complexity of the wireless devices, there is a constantneed to improve the performance of BAW resonators and BAW-based filtersand to decrease the cost and size associated therewith.

SUMMARY

An acoustic device includes a foundation structure and a transducerprovided over the foundation structure. The foundation structureincludes a piezoelectric layer between a top electrode and a bottomelectrode. The piezoelectric layer has an active portion within anactive region of the transducer and a bi-polar border portion within aborder region of the transducer. The piezoelectric material in theactive portion has a first polarization. The bi-polar border portion hasa first sub-portion and a second sub-portion, which resides either aboveor below the first sub-portion. The piezoelectric material in the firstsub-portion has the first polarization, and the piezoelectric materialin the second sub-portion has a second polarization, which is oppositethe first polarization.

Those skilled in the art will appreciate the scope of the presentdisclosure and realize additional aspects thereof after reading thefollowing detailed description of the preferred embodiments inassociation with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The accompanying drawing figures incorporated in and forming a part ofthis specification illustrate several aspects of the disclosure and,together with the description, serve to explain the principles of thedisclosure.

FIG. 1 illustrates a conventional bulk acoustic wave (BAW) resonator.

FIG. 2 is a graph of the magnitude and phase of impedance over frequencyresponses as a function of frequency for an ideal BAW resonator.

FIGS. 3A-3C are graphs of phase responses for various BAW resonatorconfigurations.

FIG. 4 illustrates a conventional BAW resonator with a border ring.

FIG. 5A is a schematic of a conventional ladder network.

FIGS. 5B and 5C are graphs of a frequency response for BAW resonators inthe conventional ladder network of FIG. 5A and a frequency response forthe conventional ladder network of FIG. 5A.

FIGS. 6A-6E are circuit equivalents for the ladder network of FIG. 5A atthe frequency points 1, 2, 3, 4, and 5, which are identified in FIG. 5C.

FIG. 7 illustrates a piezoelectric device that provides a bi-polarportion in the border region of the piezoelectric layer, according to afirst embodiment of the disclosure.

FIGS. 8A and 8B graphically illustrate the different polar orientationsof aluminum nitride.

FIG. 9A plots quality factor versus frequency shift with mass loadingfor BAW resonators with a border ring and with a bi-polar borderportion, according to one embodiment.

FIG. 9B plots border mode amplitude versus frequency shift with massloading for BAW resonators with a border ring and with a bi-polar borderportion, according to one embodiment.

FIG. 9C illustrates border mode content for BAW resonators equipped witha border ring and a bi-polar border portion, according to oneembodiment.

FIGS. 10A, 10B, and 10C illustrate lateral (spurious) mode suppressionnear the series resonance frequency (f_(s)), between the seriesresonance frequency (f_(s)) and the parallel resonance frequency(f_(p)), and near the parallel resonance frequency (f_(p)), according toone embodiment.

FIG. 11 illustrates a piezoelectric device that provides a border ringand a bi-polar portion in the border region of the piezoelectric layer,according to a second embodiment of the disclosure.

FIG. 12 illustrates a piezoelectric device that provides a bi-polarportion in the border region of the piezoelectric layer, according to athird embodiment of the disclosure.

FIG. 13 illustrates a piezoelectric device that provides a bi-polarportion in the border region of the piezoelectric layer, according to afourth embodiment of the disclosure.

FIG. 14 illustrates a piezoelectric device that provides a bi-polarportion in the border region of the piezoelectric layer, according to afifth embodiment of the disclosure.

FIG. 15 illustrates a piezoelectric device that provides a bi-polarportion in the border region of the piezoelectric layer, according to asixth embodiment of the disclosure.

DETAILED DESCRIPTION

The embodiments set forth below represent the necessary information toenable those skilled in the art to practice the embodiments andillustrate the best mode of practicing the embodiments. Upon reading thefollowing description in light of the accompanying drawing figures,those skilled in the art will understand the concepts of the disclosureand will recognize applications of these concepts not particularlyaddressed herein. It should be understood that these concepts andapplications fall within the scope of the disclosure and theaccompanying claims.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of the present disclosure. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

It will be understood that when an element such as a layer, region, orsubstrate is referred to as being “on” or extending “onto” anotherelement, it can be directly on or extend directly onto the other elementor intervening elements may also be present. In contrast, when anelement is referred to as being “directly on” or extending “directlyonto” another element, there are no intervening elements present.Likewise, it will be understood that when an element such as a layer,region, or substrate is referred to as being “over” or extending “over”another element, it can be directly over or extend directly over theother element or intervening elements may also be present. In contrast,when an element is referred to as being “directly over” or extending“directly over” another element, there are no intervening elementspresent. It will also be understood that when an element is referred toas being “connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present.

Relative terms such as “below” or “above” or “upper” or “lower” or“horizontal” or “vertical” may be used herein to describe a relationshipof one element, layer, or region to another element, layer, or region asillustrated in the Figures. It will be understood that these terms andthose discussed above are intended to encompass different orientationsof the device in addition to the orientation depicted in the Figures.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the disclosure.As used herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises,”“comprising,” “includes,” and/or “including” when used herein specifythe presence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this disclosure belongs. It willbe further understood that terms used herein should be interpreted ashaving a meaning that is consistent with their meaning in the context ofthis specification and the relevant art and will not be interpreted inan idealized or overly formal sense unless expressly so defined herein.

The present disclosure relates to a piezoelectric device with a uniquepiezoelectric layer and a method for fabricating the same. Thepiezoelectric device includes a foundation structure and a transducerprovided over the foundation structure. The foundation structureincludes a piezoelectric layer between a top electrode and a bottomelectrode. The piezoelectric layer has an active portion within anactive region of the transducer, and a bi-polar border portion within aborder region of the transducer. The piezoelectric material in theactive portion has a first polarization. The bi-polar border portion hasa first sub-portion and a second sub-portion, which resides either aboveor below the first sub-portion. The piezoelectric material in the firstsub-portion has the first polarization, and the piezoelectric materialin the second sub-portion has a second polarization, which is oppositethe first polarization.

The piezoelectric device may be implemented in a variety of devices,such as a bulk acoustic wave (BAW) resonator. Prior to delving into thedetails of the unique piezoelectric film, an overview of a BAW resonatorand its operation is described. BAW resonators are used in manyhigh-frequency filter applications. An exemplary BAW resonator 10 isillustrated in FIG. 1. The BAW resonator 10 is a solidly mountedresonator-type BAW resonator 10 and generally includes a substrate 12, areflector 14 mounted over the substrate 12, and a transducer 16 mountedover the reflector 14. The transducer 16 rests on the reflector 14 andincludes a piezoelectric layer 18, which is sandwiched between a topelectrode 20 and a bottom electrode 22. The top and bottom electrodes 20and 22 may be formed of tungsten (W), molybdenum (Mo), platinum (Pt), orlike material, and the piezoelectric layer 18 may be formed of aluminumnitride (AlN), zinc oxide (ZnO), or other appropriate piezoelectricmaterial. Although shown in FIG. 1 as including a single layer, thepiezoelectric layer 18, the top electrode 20, and/or the bottomelectrode 22 may include multiple layers of the same material, multiplelayers in which at least two layers are different materials, or multiplelayers in which each layer is a different material. The combination ofthe substrate 12 and the reflector 14 is generally referred to as afoundation structure for the BAW resonator 10, and as such thetransducer 16 resides on or over the foundation structure.

Continuing with FIG. 1, the BAW resonator 10 is divided into an activeregion 24 and an outside region 26. The active region 24 generallycorresponds to the section of the BAW resonator 10 where the top andbottom electrodes 20 and 22 overlap and also includes the layers belowthe overlapping top and bottom electrodes 20 and 22. The outside region26 corresponds to the section of the BAW resonator 10 that surrounds theactive region 24. The portion of the piezoelectric layer 18 that residesin the active region 24 is referred to as an active portion SA, and theportion of the piezoelectric layer 18 that resides in the outside region26 is referred to as an outer portion SO.

For the BAW resonator 10, applying electrical signals across the topelectrode 20 and the bottom electrode 22 excites acoustic waves in thepiezoelectric layer 18. These acoustic waves primarily propagatevertically. A primary goal in BAW resonator design is to confine thesevertically propagating acoustic waves within the transducer 16. Acousticwaves traveling upwardly are reflected back into the transducer 16 bythe air-metal boundary at a top surface of the top electrode 20.Acoustic waves traveling downwardly are reflected back into thetransducer 16 by the reflector 14 or by an air cavity (not shown), whichis provided just below the transducer in a film BAW resonator (FBAR).

The reflector 14 is typically formed by a stack of reflector layers (RL)28, which alternate in material composition to produce a significantreflection coefficient at the junction of adjacent reflector layers 28.Typically, the reflector layers 28 alternate between materials havinghigh and low acoustic impedances, such as tungsten (W) and silicondioxide (SiO₂). While only five reflector layers 28 are illustrated inFIG. 1, the number of reflector layers 28 and the structure of thereflector 14 vary from one design to another.

The magnitude (Z) and phase (4)) of the electrical impedance as afunction of the frequency for a relatively ideal BAW resonator 10 isprovided in FIG. 2. The magnitude (Z) of the electrical impedance isillustrated by the solid line, while the phase (4)) of the electricalimpedance is illustrated by the dashed line. A unique feature of the BAWresonator 10 is that it has both a resonance frequency and ananti-resonance frequency. The resonance frequency is typically referredto as the series resonance frequency (f_(s)), and the anti-resonancefrequency is typically referred to as the parallel resonance frequency(f_(p)). The series resonance frequency (f_(s)) occurs when themagnitude of the impedance, or reactance, of the BAW resonator 10approaches zero. The parallel resonance frequency (f_(p)) occurs whenthe magnitude of the impedance, or reactance, of the BAW resonator 10peaks at a significantly high level. In general, the series resonancefrequency (f_(s)) is a function of the thickness of the piezoelectriclayer 18 and the mass of the bottom and top electrodes 20 and 22.

For the phase, the BAW resonator 10 acts like an inductance thatprovides a 90° phase shift between the series resonance frequency(f_(s)) and the parallel resonance frequency (f_(p)). In contrast, theBAW resonator 10 acts like a capacitance that provides a −90° phaseshift below the series resonance frequency (f_(s)) and above theparallel resonance frequency (f_(p)). The BAW resonator 10 presents avery low, near zero, resistance at the series resonance frequency(f_(s)) and a very high resistance at the parallel resonance frequency(f_(p)). The electrical nature of the BAW resonator 10 lends itself tothe realization of a very high Q (quality factor) inductance over arelatively short range of frequencies, which has proved to be verybeneficial in high frequency filter networks, especially those operatingat frequencies around 1.8 GHz and above.

Unfortunately, the phase (4)) curve of FIG. 2 is representative of anideal phase curve. In reality, approaching this ideal is challenging. Atypical phase curve for the BAW resonator 10 of FIG. 1 is illustrated inFIG. 3A. Instead of being a smooth curve, the phase curve of FIG. 3Aincludes ripple below the series resonance frequency (f_(s)), betweenthe series resonance frequency (f_(s)) and the parallel resonancefrequency (f_(p)), and above the parallel resonance frequency (f_(p)).The ripple is the result of spurious modes, which are caused by spuriousresonances that occur in corresponding frequencies. While the vastmajority of the acoustic waves in the BAW resonator 10 propagatevertically, various boundary conditions about the transducer 16 resultin the propagation of lateral (horizontal) acoustic waves, which arereferred to as lateral standing waves. The presence of these lateralstanding waves reduces the potential Q associated with the BAW resonator10.

As illustrated in FIG. 4, a border (BO) ring 30 is formed on or withinthe top electrode 20 to suppress certain of the spurious modes. Thespurious modes that are suppressed by the BO ring 30 are those above theseries resonance frequency (f_(s)), as highlighted by circles A and B inthe phase curve of FIG. 3B. Circle A shows a suppression of the ripple,and thus the spurious mode, in the passband of the phase curve, whichresides between the series resonance frequency (f_(s)) and the parallelresonance frequency (f_(p)). Circle B shows suppression of the ripple,and thus the spurious modes, above the parallel resonance frequency(f_(p)). Notably, the spurious mode in the upper shoulder of thepassband, which is just below the parallel resonance frequency (f_(p)),and the spurious modes above the passband are suppressed, as evidencedby the smooth or substantially ripple-free phase curve between theseries resonance frequency (f_(s)) and the parallel resonance frequency(f_(p)) and above the parallel resonance frequency (f_(p)).

The BO ring 30 corresponds to a mass loading of the portion of the topelectrode 20 that extends about the periphery of the active region 24.The BO ring 30 may correspond to a thickened portion of the topelectrode 20 or the application of additional layers of an appropriatematerial over the top electrode 20. The portion of the BAW resonator 10that includes and resides below the BO ring 30 is referred to as aborder (BO) region 32. The portion of the piezoelectric layer 18 withinthe BO region 32 is referred to as a border portion SB. Accordingly, theBO region 32 generally resides between the active region 24 and theoutside region 26, and the border portion SB resides between the activeportion SA and the outer portion SO.

While the BO ring 30 is effective at suppressing spurious modes abovethe series resonance frequency (f_(s)), the BO ring 30 generally hasmuch less impact on those spurious modes below the series resonancefrequency (f_(s)), as shown in FIG. 3B. A technique referred to asapodization is often used to suppress the spurious modes that fall belowthe series resonance frequency (f_(s)).

Apodization works to reduce any lateral symmetry in the BAW resonator10, or at least in the transducer 16 thereof. The lateral symmetrycorresponds to the footprint of the transducer 16, and avoiding thelateral symmetry corresponds to avoiding symmetry associated with thesides of the footprint. For example, one may choose a footprint thatcorresponds to a pentagon instead of a square or rectangle. Avoidingsymmetry helps reduce the presence of lateral standing waves in thetransducer 16. Circle C of FIG. 3C illustrates the effect of apodizationin which the spurious modes below the series resonance frequency (f_(s))are suppressed. Assuming no BO ring 30 is provided, one can readily seein FIG. 3C that apodization fails to suppress those spurious modes abovethe series resonance frequency (f_(s)). As such, the typical BAWresonator 10 employs both apodization and the BO ring 30.

As noted previously, BAW resonators 10 are often used in filter networksthat operate at high frequencies and require high Q values. A basicladder network LN is illustrated in FIG. 5A. The ladder network LNincludes two series resonators B_(SER) and two shunt resonators B_(SH),which are arranged in a traditional ladder configuration. Typically, theseries resonators B_(SER) have the same or similar first frequencyresponse, and the shunt resonators B_(SH) have the same or similarsecond frequency response, which is different from the first frequencyresponse, as shown in FIG. 5B. In many applications, the shuntresonators B_(SH) are a detuned version of the series resonatorsB_(SER). As a result, the frequency responses for the series resonatorsB_(SER) and the shunt resonators B_(SH) are generally very similar, yetshifted relative to one another such that the parallel resonancefrequency (f_(p,SH)) of the shunt resonators approximates the seriesresonance frequency (f_(s,SER)) of the series resonators B_(SER). Notethat the series resonance frequency (f_(s,SH)) of the shunt resonatorsB_(SH) is less than the series resonance frequency (f_(s,SER)) of theseries resonators B_(SER). The parallel resonance frequency (f_(p,SH))of the shunt resonators B_(SH) is less than the parallel resonancefrequency (f_(p,SER)) of the series resonators B_(SER).

FIG. 5C is associated with FIG. 5B and illustrates the response of theladder network LN. The series resonance frequency (f_(s,SH)) of theshunt resonators B_(SH) corresponds to the low side of the passband'sskirt (phase 2), and the parallel resonance frequency (f_(p,SER)) of theseries resonators B_(SER) corresponds to the high side of the passband'sskirt (phase 4). The substantially aligned series resonance frequency(f_(s,SER)) of the series resonators B_(SER) and the parallel resonancefrequency (f_(p,SH)) of the shunt resonators B_(SH) fall within thepassband. FIGS. 6A through 6E provide circuit equivalents for the fivephases of the response of the ladder network LN. During the first phase(phase 1, FIGS. 5C, 6A), the ladder network LN functions to attenuatethe input signal. As the series resonance frequency (f_(s,SH)) of theshunt resonators B_(SH) is approached, the impedance of the shuntresonators B_(SH) drops precipitously, such that the shunt resonatorsB_(SH) essentially provide a short to ground at the series resonancefrequency (f_(s,SH)) of the shunt resonators (phase 2, FIGS. 5C, 6B). Atthe series resonance frequency (f_(s,SH)) of the shunt resonators B_(SH)(phase 2), the input signal is essentially blocked from the output ofthe ladder network LN.

Between the series resonance frequency (f_(s,SH)) of the shuntresonators B_(SH) and the parallel resonance frequency (f_(p,SER)) ofthe series resonators B_(SER), which corresponds to the passband, theinput signal is passed to the output with relatively little or noattenuation (phase 3, FIGS. 5C, 6C). Within the passband, the seriesresonators B_(SER) present relatively low impedance, while the shuntresonators B_(SH) present a relatively high impedance, wherein thecombination of the two leads to a flat passband with steep low-side andhigh-side skirts. As the parallel resonance frequency (f_(p,SER)) of theseries resonators B_(SER) is approached, the impedance of the seriesresonators B_(SER) becomes very high, such that the series resonatorsB_(SER) essentially present themselves as an open at the parallelresonance frequency (f_(p,SER)) of the series resonators (phase 4, FIGS.5C, 6D). At the parallel resonance frequency (f_(p,SER)) of the seriesresonators B_(SER) (phase 4), the input signal is again essentiallyblocked from the output of the ladder network LN. During the final phase(phase 5, FIGS. 5C, 6E), the ladder network LN functions to attenuatethe input signal in a similar fashion to that provided in phase 1. Asthe parallel resonance frequency (f_(p,SER)) of the series resonatorsB_(SER) is passed, the impedance of the series resonators B_(SER)decreases, and the impedance of the shunt resonators B_(SH) normalizes.Thus, the ladder network LN functions to provide a high Q passbandbetween the series resonance frequency (f_(s,SH)) of the shuntresonators B_(SH) and the parallel resonance frequency (f_(p,SER)) ofthe series resonators B_(SER). The ladder network LN provides extremelyhigh attenuation at both the series resonance frequency (f_(s,SH)) ofthe shunt resonators B_(SH) and the parallel resonance frequency(f_(p,SER)) of the series resonators. The ladder network LN providesgood attenuation below the series resonance frequency (f_(s,SH)) of theshunt resonators B_(SH) and above the parallel resonance frequency(f_(p,SER)) of the series resonators B_(SER).

In a single modern communication system, such as a mobile telephone,numerous filters require passbands of different bandwidths and centeredat different frequencies. The center frequencies of filters that employBAW resonators 10 are primarily governed by the thicknesses of thevarious layers of the transducer 16 and, in particular, the thickness ofthe piezoelectric layer 18. The passband bandwidths and shapes of theband edges of the filters are primarily governed by theelectromechanical coupling coefficient k of the piezoelectric layer 18.An electromechanical coupling coefficient k is the measure of theeffectiveness of the piezoelectric layer in converting electrical energyto mechanical energy, and vice versa. Different piezoelectric materialsor material compositions generally have different electromechanicalcoupling coefficients k.

For passbands having bandwidths less than 100 MHz, aluminum nitride(AlN) is a common choice for the piezoelectric layer 18. For passbandhaving bandwidth greater than 100 MHz, newer piezoelectric materialsthat provide an increased electromechanical coupling coefficient k arebeing employed. These newer piezoelectric materials include, but are notlimited to, aluminum nitride that has been doped with one or moretransition metals, such as scandium (Sc), yttrium (Y), magnesium (Mg),zirconium (Zr), and the like, alone or in combination with othermaterials such as erbium (Er), magnesium (Mg), and the like. Exemplarypiezoelectric materials include, but are not limited to ScAlN, YAlN,[Mg][Zr]AlN, [Sc][Er]AlN, and the like.

Unfortunately, each of these piezoelectric materials has a fairlyspecific electromechanical coupling factor k. As a result, designerscurrently have to pick a particular piezoelectric material and thendesign the rest of the BAW resonator 10 and the filters that employ theBAW resonator 10 around the electromechanical coupling factor k of thechosen piezoelectric material. In other words, the choice of thepiezoelectric material for the piezoelectric layer 18 restricts theelectromechanical coupling factor k and, as such, ultimately limits theability of the designer to optimize the performance of the overallfilter design. Further, designers would benefit from a technique forproviding electromechanical coupling in certain areas of thepiezoelectric layer 18 and providing essentially zero electromechanicalcoupling in other areas of the piezoelectric layer 18. For example, onewould like to provide electromechanical coupling at a desired level inthe active regions 24 of the BAW resonators 10 and little or noelectromechanical coupling in the outside regions 26 and/or BO regions32.

The electromechanical coupling factor of a material is a function of thepiezoelectric properties of the material. As such, non-piezoelectricmaterials exhibit little or no electromechanical coupling and thus havean electromechanical coupling factor k of zero or approaching zero. Thepiezoelectric materials exhibit an electromechanical coupling factor kbased at least in part on the piezoelectric properties of the material.

The following describes a technique for providing both piezoelectric andnon-piezoelectric portion, or regions, in the piezoelectric layer 18. Asdescribed above, multiple BAW resonators 10 are often used inconjunction to form ladder networks LN and the like. In many instances,the multiple BAW resonators 10 that are used to form the ladder networksLN are integrated on a single die, wherein the transducers 16 of thedifferent BAW resonators 10 share a common substrate 12, reflector 14,and the like. Further, the piezoelectric layers 18, top electrodes 20,and bottom electrodes 22 are individually formed from common materiallayers through appropriate deposition and etching processes.

With reference to FIG. 7, the present disclosure relates to apiezoelectric layer 18 in which the portion of the piezoelectric layer18 that resides in the BO region 32 is bi-polar. This bi-polar portionis referred to as a bi-polar border portion BSB. In most embodiments,the active portion SA in the active region 24 remains unipolar. Theouter portion SO in the outside region 26 may be unipolar or bi-polar indifferent embodiments. A piezoelectric material has intrinsic polarorientation, and the direction of polar orientation for one embodimentis shown by arrows in FIG. 7. For BAW resonators, the verticalpolarizations (‘up’ or ‘down’) are significant for electromechanicalconversion. Within the active portion SA and the outer portion SO of thepiezoelectric layer 18, the polar orientation is constant throughout therespective portions and as such is unipolar.

In contrast, the bi-polar border portion BSB of the piezoelectric layer18 is formed to have sub-portions 34, 36, which have opposingpolarizations, as indicated by the opposing arrows within thoseportions. The sub-portions 34, 36 of the bi-polar border portion BSB maybe formed from the same material as the rest of the piezoelectric layer18 but are formed to have opposing polarizations. An inversion layer ILrepresents the border between the sub-portions 34, 36 of the bi-polarborder portion BSB. The inversion layer IL may simply represent apolarization transition level or an actual structure of one or morelayers that actually triggered the polarization reversal duringfabrication, which is described in further detail subsequently.

Since the polarity change does not affect the acoustic properties of thepiezoelectric layer, the active portion SA and bi-polar border portionSB are acoustically similar and there is essentially no additional massloading within bi-polar border portion BSB of the piezoelectric layer 18caused by making border portion SB bi-polar. As a result, the bi-polarborder portion BSB confines the lateral acoustic energy with or withoutthe need for the mass loading, thereby potentially eliminating the needfor a BO ring 30, which is not present in the embodiment of FIG. 7. Theresulting quality factor Q is better or comparable to values realized bydesigns that incorporate BO rings 30.

Incorporation of the bi-polar border portion BSB does not generateborder (BO) modes in the vicinity of active area series resonancefrequency (f_(s)). “Vicinity” is defined as being within 300 MHz of theseries resonance frequency (f_(s)). Instead, the BO mode shifts toseveral thousands of megahertz above the active area series resonancefrequency (f_(s)), thereby having virtually no effect on performance offilters that employ these BAW resonators 10. Use of the bi-polar borderportion BSB also reduces the spurious lateral modes better or comparableto the conventional designs that incorporate BO rings 30. Further detailon these and other benefits is provided subsequently.

Common piezoelectric materials for the piezoelectric layer 18 includegroup III-V nitrides, primarily aluminum nitride (AlN) and AlN dopedwith transition metals such as Sc, Er, Mg, Hf, and the like. If thepiezoelectric layer 18 is formed from AlN, the direction of the Al—Nbond along the c-axis of the AlN crystalline structure determines thepolarization, or polarity direction, of the AlN crystalline structure.The polarization of the material can be inverted by flipping thedirection of Al—N bond during growth of the piezoelectric layer 18.Several processing methods exist to achieve polarization inversion ofIII-V nitrides. For example, AlN grows with N-polarity (N-polar) on andover certain metal surfaces, such as W, Cu, and Mo, while it grows withAl-polarity (Al-polar) on and over surfaces such as Al, Ru, and RuO₂. Anappropriate inversion layer IL may be deposited and patternedlithographically in the BO region 32 during the growth process for thepiezoelectric layer 18 to achieve the bi-polar border portion BSBillustrated in FIG. 7.

For the illustrated embodiment, a solidly mounted resonator or FBARfoundation structure is provided. The bottom electrode 22 is formed overthe foundation structure by depositing an appropriate metal layer or thelike over the piezoelectric layer foundation structure and then etchingthe metal layer in a manner leaving the bottom electrode 22. Next, alower portion of the piezoelectric layer 18 is grown with a firstpolarization to a first thickness over the bottom electrode 22 andthrough the outside region 26, the BO region 32, and the active region24. At this level, an appropriate metal layer or the like is depositedover the piezoelectric layer 18 and then etched in a manner leaving theinversion layer IL. As noted previously, the inversion layer IL residesin the BO region 32.

Next, the growth process for the piezoelectric layer 18 resumes suchthat an upper portion of the piezoelectric layer 18 is grown over thelower portion of the piezoelectric layer 18 across the outside region26, BO region 32, and active region 24 until the piezoelectric layer 18reaches a second thickness. Notably, the polarization of piezoelectriclayer 18 that is grown over the inversion layer IL is inverted, and assuch, the piezoelectric layer 18 over the inversion layer IL and withinthe BO region 32 has a second polarization, which is opposite that ofthe first polarization. The polarization of the piezoelectric layer 18that is grown in the active portion SA and the outer portion SO is notinverted, and as such, has the first polarization. The top electrode 20is formed over the piezoelectric layer 18 by depositing an appropriatemetal layer or the like over the piezoelectric layer and then etchingthe metal layer in a manner leaving the top electrode 20.

FIG. 8A illustrates an aluminum polar (Al-polar) orientation wherein thegrowth direction is along the c-axis of the atomic structure. For anAl-polar atomic structure, the Al—N atomic bond along c-axis is suchthat Al atom resides below the N atom. The net polarity, P_(NET), isinverted, as represented by the downward arrow. The opposite is true fora nitrogen polar (N-polar) orientation with growth direction is alongthe c-axis, as illustrated in FIG. 8B. For a N-polar atomic structure,the Al—N atomic bond along c-axis is such that N atom resides below theAl atom. The net polarity, P_(NET), is aligned with the growthdirection, as represented by the upward arrow. As such, the polarizationof the Al-polar orientation is opposite that of the N-polar orientation.While aluminum nitride is used in this example, other piezoelectriccompounds may be used for the piezoelectric material and an appropriatemetal, such as the metal of the compound, may be used to form theinversion layer IL.

The ratio of the thicknesses of the sub-portions 34, 36, which haveopposite polarization, may be chosen to cancel the fundamental BAW modein the BO region 32. The thicknesses of the respective layers may be,but need not be, 1:1. The particular ratio may depend on the stack ofmaterials and elements beneath and above the piezoelectric layer 18.Typically, the ratio ranges from 0.7:1 to 1.3:1 for most of the BAWresonators, while other embodiments may require a tighter range of 0.8:1to 1.2:1, 0.9:1 to 1.1:1, 0.95:1 to 1.05:1, and the like. Other broaderor tighter ranges are possible and are considered to be within the scopeof this disclosure. While the bi-polar border portion BSB is shown toswitch polarization only once, multiple polarity switches (i.e.,alternating two or more of each sub-portion 34, 36) throughout thethickness of piezoelectric layer 18 is also envisioned, as describedfurther subsequently. The aforementioned thickness ratios also apply forthe cumulative thicknesses of the respective sub-portions 34, 36.

When using a convention BO ring 30, as illustrated in FIG. 4, the extramass loading that is provided by the BO ring 30 in the BO region 32introduces a downward frequency shift in the BO region 32 of theflexural BAW mode. The resulting change in acoustic dispersion betweenthe active region 24 and the BO region 32 is exploited to confine thelateral acoustic energy within the active portion SA of thepiezoelectric layer 18. FIG. 9A plots the downshift in frequency vs. thequality factor Qp at optimal BO ring 30 width. The magnitude of the massloading provided by the BO ring 30 is critical to achieve better Qp.

For the present disclosure, the bi-polar border portion BSB confines theenergy of the lateral waves, without a need for significant change inacoustic dispersion from the active portion SA. In FIG. 9A,significantly higher Qp values are achieved with negligible massloading. Though polarization inversion does not induce changes inmechanical dispersion, the bi-polar border portion BSB does not supportseveral modes due to the electrical boundary conditions. Theanti-symmetric modes (in z-direction) in the active portion SA do notpropagate into the bi-polar border portion BSB and thus remain confinedwithin the active portion SA.

The active region 24 of BAW resonators resonates at the fundamentalseries resonance frequency (f_(s)). A novel aspect of the bi-polarborder portion BSB is that it does not support the fundamental acoustictones. Fundamental mode resonances have zero coupling (k2e) instructures where the sub-portions 34, 36 have inverted polarizations.Irrespective of the mass loading added to or by the bi-polar borderportion BSB, no modes in vicinity of the series resonance frequency(f_(s)) are excited in the BO region 32. As a result, no modes aregenerated from the BO region 32 at the frequencies that are critical fora filter made of such BAW resonators 10. FIG. 9B shows that BO modes areeffectively suppressed in the BO region 32 when the BO region 32 isdesigned as a bi-polar border portion BSB. At the fundamental seriesresonance frequency (f_(s)), since bi-polar BO region 32 does notparticipate in the electromechanical conversion, the effectivepiezoelectric coupling may decrease by up to 4% or more.

The bi-polar border portion BSB does support second overtone resonances.These resonances typically occur at frequencies that are greater thantwo times the fundamental series resonance frequency (f_(s)). Since mostBAW resonators 10 operate at frequencies above 1500 MHz, the secondovertones from the BO region 32 occur at least thousands of megahertzfrom the fundamental series resonance frequency (f_(s)). FIG. 9C showsthe BO mode content for BAW resonators 10 equipped with a BO ring 30 anda bi-polar border portion BSB, respectively. As illustrated, using thebi-polar border portion BSB solves the issues of BO modes falling withinthe filter passband and enables better design of BAW filters of anybandwidth. In addition, use of the bi-polar border portion BSBfacilitates multiplexing with closely spaced bands, as is often requiredfor duplexers and multiplexers.

Further, use of the bi-polar border portion BSB to define the BO region32 offers much better lateral (spurious) mode suppression than use of aBO ring 30 alone. FIGS. 10A, 10B and 10C compare the best-case scenariosfor both the configurations. The spurious content is shown just belowseries resonance frequency (f_(s)), between series resonance frequency(f_(s)) to parallel resonance frequency (f_(p)), and just above parallelresonance frequency (f_(p)), respectively, in FIGS. 10A, 10B and 10C.These three frequency regions are of prime importance in shaping thefilter passband. Note that the design with the bi-polar border portionBSB performs better than the design with a BO ring 30 in suppressingspurious content in all the three frequency regimes. BAW resonators 10that incorporate the bi-polar border portion BSB have a potential toallow filters made therefrom to achieve spurious-mode-free passbands.

FIG. 11 illustrates a BAW resonator 10 that incorporates a BO ring 30over the bi-polar border portion BSB of the BO region 32. FIG. 12illustrates a BAW resonator 10 that incorporates an inversion layer ILthat is thick enough to provide a mass loading within the bi-polarborder portion BSB. As such, the inversion layer IL in this embodimentis a thickened structure of one or more layers with the desired mass andprovided between the sub-portions 34, 36 within the bi-polar borderportion BSB. The inversion layer IL in this embodiment may take theplace of and provide the mass loading of a BO ring 30. Further, theinversion layer IL may be provided in conjunction with a BO ring 30 thatresides on top of the bi-polar portion BSB.

Any of the foregoing embodiments may be implemented in an FBARconstruction, wherein the reflector 14 is essentially replaced with anair gap 38, as illustrated in FIG. 13. The foundation structure for theFBAR embodiment includes the substrate 12 and the air gap 38. As such,the transducer 16 essentially resides on or over the foundationstructure. In this embodiment, a BO ring 30 and an inversion layer IL,which may be configured to add additional mass, are provided.

There are many potential alternatives to the embodiments describedpreviously. For example, the sub-portions 34, 36 of the BO region 32 mayswap polarizations relative to the above-described embodiments, asillustrated in FIG. 14. As indicated previously, the bi-polar borderportion BSB may have more than two sub-portions 34, 36. As illustratedin FIG. 15, the bi-polar border portion BSB is divided into foursub-portions 40, 42, 44, 46, which are separated by three inversionlayers IL. The inversion layers IL facilitate the polarization swappingof the piezoelectric material within the bi-polar border portion BSBduring the growth or deposition process for the piezoelectric layer 18.Sub-portions 40, 44 have a first polarization, and sub-portions 42, 46have a second polarization, which is opposite of the first polarization.In this embodiment, the outer portion OS and the active portion SA alsohave the second polarization; however, all of these polarizations couldbe swapped. Further, the polarizations of the sub-portions 40, 42, 44,46 could be swapped. The number and the thicknesses of the sub-portions40.42, 44, 46 may vary from one embodiment to another depending on theperformance metrics required for the application.

Those skilled in the art will recognize improvements and modificationsto the preferred embodiments of the present disclosure. All suchimprovements and modifications are considered within the scope of theconcepts disclosed herein and the claims that follow.

What is claimed is:
 1. An piezoelectric device comprising: a foundationstructure; a transducer over the foundation structure and comprising: abottom electrode; a piezoelectric layer over the bottom electrode andcomprising a piezoelectric material; and a top electrode over thepiezoelectric layer, wherein: the piezoelectric layer has an activeportion within an active region of the transducer, and a bi-polar borderportion within a border region of the transducer; the piezoelectricmaterial in the active portion has a first polarization; the bi-polarborder portion comprises a first sub-portion and a second sub-portionthat is either over or under the first sub-portion; and thepiezoelectric material in the first sub-portion has the firstpolarization, and the piezoelectric material in the second sub-portionhas a second polarization, which is opposite the first polarization. 2.The piezoelectric device of claim 1 further comprising a first inversionlayer between the first sub-portion and the second sub-portion.
 3. Thepiezoelectric device of claim 2 wherein the first inversion layerprovides mass loading within the border region.
 4. The piezoelectricdevice of claim 3 further comprising a border ring within the borderregion and over the piezoelectric layer wherein the border ring providesmass loading.
 5. The piezoelectric device of claim 1 further comprisinga border ring within the border region and over the piezoelectric layerwherein the border ring provides mass loading.
 6. The piezoelectricdevice of claim 1 wherein the piezoelectric material is formed from acompound comprising a metal element and a non-metal element.
 7. Thepiezoelectric device of claim 4 wherein the metal element is a group IIIelement and the non-metal element is a group V element.
 8. Thepiezoelectric device of claim 1 wherein the piezoelectric materialcomprises AlN doped with a transition metal selected from the groupscandium, erbium, magnesium, and hafnium.
 9. The piezoelectric device ofclaim 1 wherein the piezoelectric material has an outer portion withinan outside region such that the border region is between the outsideregion and the active region.
 10. The piezoelectric device of claim 1wherein the piezoelectric material in the outer portion has the firstpolarization.
 11. The piezoelectric device of claim 1 wherein thefoundation structure comprises a substrate and a reflector, whichcomprises a plurality of reflector layers, over the substrate, such thatthe device is a solidly mounted resonator-based bulk acoustic waveresonator.
 12. The piezoelectric device of claim 1 wherein thefoundation structure comprises a substrate that provides an air gapbelow the transducer, such that the device is a film bulk acousticresonator (FBAR).
 13. The piezoelectric device of claim 1 wherein thebi-polar border portion comprises at least two additional sub-portionseither over or under the first sub-portion and the second sub-portion,the at least two additional sub-portions alternating between the firstpolarization and the second polarization.
 14. The piezoelectric deviceof claim 1 wherein an outer portion of the piezoelectric layer has onlyone of the first polarization and the second polarization.
 15. Thepiezoelectric device of claim 1 wherein a ratio of a thickness of thefirst sub-portion to a thickness of the second sub-portion is between0.7:1.0 and 1.3:1.0.
 16. A method for forming a piezoelectric devicecomprising: providing a foundation structure; forming a bottom electrodeover the foundation structure; forming a lower portion of apiezoelectric layer over the bottom electrode and through an outsideregion, a border region, and an active region, wherein the active regionis inside of the border region and the border region is inside of theoutside region; within the border region, forming an inversion layerover the lower portion of the piezoelectric layer; forming an upperportion of the piezoelectric layer over the inversion layer and throughthe outside region, the border region, and the active region, wherein:the lower portion of the piezoelectric layer below the inversion layerand within the border region has a first polarization, and the upperportion of piezoelectric layer over the inversion layer and within theborder region has a second polarization, which is opposite that of thefirst polarization; and forming a top electrode over the upper portionof the piezoelectric layer.
 17. The method of claim 16 wherein theactive portion of the piezoelectric has only one of the firstpolarization and the second polarization.
 18. The method of claim 16wherein an active portion and the outer portion of the piezoelectriclayer each has only one of the first polarization and the secondpolarization.
 19. The method of claim 16 wherein an outer portion of thepiezoelectric layer has only one of the first polarization and thesecond polarization.
 20. The method of claim 16 wherein the foundationstructure comprises a substrate and a reflector, which comprises aplurality of reflector layers, over the substrate, such that the deviceis a solidly mounted resonator-based bulk acoustic wave resonator. 21.The method of claim 16 wherein the foundation structure comprises asubstrate that provides an air gap below a transducer, such that thedevice is a film bulk acoustic resonator (FBAR).
 22. The method of claim16 wherein a first inversion layer provides mass loading within theborder region.
 23. The method of claim 16 wherein a ratio of a thicknessof a first sub-portion to a thickness of a second sub-portion is between0.7:1 and 1.3:1.0.